Conventionally, substances poorly soluble in carbon dioxide in a supercritical state are not capable of being atomized by an atomization method using carbon dioxide in a supercritical state. It is because the conventional atomization method using carbon dioxide in a supercritical state requires dissolving the substances to be atomized in supercritical carbon dioxide fluid as the condition thereof.
A conventional apparatus of forming fine particles by utilizing carbon dioxide in a supercritical state is equipped with mixed solvent preparing means for mixing liquefied carbon dioxide and a dissolution-facilitating solvent, in a predetermined proportion of the former to the latter to prepare a mixed solvent; solution preparing means for making the mixed solvent in a supercritical state and dissolving a substance in the mixed solvent; and spraying means, communicating with the solution preparing means, for spraying the solution prepared in the solution preparing means. (See claim 1 of Patent document 1.)
Reference 1 includes the following explanation about the ‘dissolution-facilitating solvent’ used in the apparatus: “The dissolution-facilitating solvent is a solvent that is added to carbon dioxide to enhance the solubility of a substance poorly soluble in carbon dioxide in a supercritical state. The use of a dissolution-facilitating solvent to improve solubility of a substance is known. If a substance has a larger solubility in a mixed solvent of carbon dioxide in a supercritical state and an added solvent than in sole carbon dioxide in a supercritical state, the added solvent could be a dissolution-facilitating solvent. The dissolution-facilitating solvent may be called ‘solvent aid’, because it helps a substance dissolve in carbon dioxide in a supercritical state. Examples of the dissolution-facilitating solvent may include acetone, methanol and methylene chloride. A solvent appropriate for a substance to be dissolved is properly selected and used.” (See paragraph 0019 of Patent document 1.)
The apparatus of preparing fine particles disclosed in Patent document 1 is considered to be appropriate to atomization of medicaments. (See paragraphs 0002 and 0025 of Reference 1.)
Patent document 2 discloses “a method of atomization comprising mixing carbon dioxide to be brought into a supercritical state and a solvent with a collected polymer; making the carbon dioxide in a supercritical state; keeping the carbon dioxide in the supercritical state for a predetermined time period; reducing the pressure, thereby removing the carbon dioxide; collecting a remaining liquid; and drying the liquid”. (See claim 1 of Patent document 2.)
Patent document 2 states: “Examples of the solvent may include ethanol, n-butyl alcohol, THF which stands for tetrahydrofuran and should be understood as it hereinafter, xylene, etc. These solvents may be used singly, or two or more of them may be used together.” (See paragraph 0009 of Patent document 2.)
The apparatuses disclosed in these patent documents use an organic solvent in addition to carbon dioxide, to atomize a substance with low solubility in carbon dioxide in a supercritical state. The organic solvent used with carbon dioxide is effused into the atmosphere when the pressure vessel containing a supercritical fluid is opened to the atmosphere. The use of an organic solvent is thus problematic from the viewpoint of environmental pollution. Collection of the used organic solvent in order to solve the problem of environmental pollution requires many steps, which makes the collection unpractical.    Patent document 1: Japanese Patent No. 3754372    Patent document 2: JP 2006-111798 A